A process and a liquefier have been disclosed in the article in Process Engineering (March 1997) "The Air that I Breathe." Air is liquefied in a low-temperature process, subjected to low-temperature rectification, and a liquid air product with an oxygen content of between 16.5 and 21% oxygen is produced. This air is produced by mixing an oxygen product and a nitrogen product (Synthetic Liquid Air, SLA). In this system, it is disadvantageous that there is a waste of energy to separate the air into liquid products of oxygen and nitrogen which are recombined to form SLA.